Electromagnetic waves, such as microwaves, can have a polarization, such as a linear polarization. The polarization of linearly polarized electromagnetic waves can be characterized by a polarization angle. A polarization modulator operates on these linearly polarized waves to cause a change, or rotation, of the polarization angle. In the prior art, polarization modulation at millimeter-wavelengths (microwave) has been done by physical rotation of mechanical devices, such as by mechanical rotation of a waveguide polarizer, rotation of a wire grid polarizer, or rotation of a birefringent half-waveplate or mechanical rotation of a dielectric card in a waveguide.
Brian Keating (one of the present inventors) previously designed a polarization modulating device having no moving parts, based on the principles of Faraday rotation. The device was built in a smooth walled waveguide structure and was only capable of operating at specific microwave frequencies (over a very narrow bandwidth). Keating's first Faraday rotation device described above, albeit without moving parts, proved unsuitable for cosmic microwave background (“CMB”) polarimeter applications because of the narrow band operation.
Measurements of the polarization of the CMB have the promise to revolutionize our understanding of the early universe. Unlike the temperature anisotropy of the CMB which has been measured to relatively high precision over a wide range of angular scales, the polarization of the CMB has only recently been detected and remains relatively unexplored. Polarimeters potentially useful for such studies have typically employed mechanical mechanisms to modulate the incident CMB radiation field about an optical axis.
In conjunction with an analyzer (to decompose the radiation into orthogonal polarization states), a polarization modulator can be used to exchange the polarized intensity between the two detectors or amplifiers (or to switch the polarization incident on a single detector). If the modulation is done rapidly enough, this technique is useful to mitigate the effects of detector gain instability. Since polarization measurements are often low signal-to-noise, modulation, without such mitigation, gain and offset instability can masquerade as polarization.
It can be advantageous to operate a polarimeter polarization modulator at as high a speed as possible. Prior art mechanical modulators were only capable of modulation rates up to 100 Hz, if that high, as limited for example, by mechanically rotating plates. The problem is that while a modulation frequency of 100 Hz can mitigate the affects of some detector gain and offset variations, it does not help for higher frequency changes in gain and offset, nor is it fast enough to attenuate 1/f noise caused by electronic amplifiers, such as detector and difference amplifiers, used in most polarimeters. Another problem with mechanical polarization modulators of the prior art is that the mechanical vibration from moving elements can introduce a false electrical signal related caused by mechanical vibration.
What is needed is a polarization modulator having no moving mechanical components that can continuously vary an angle of polarization modulation over a wide bandwidth, and that can operate at modulation frequencies over 100 Hz and over a broad band.